Method for the treatment of water and wastewater

ABSTRACT

A method and a plant for the treatment of water or wastewater, having impurities therein by filtration through two granular media filter stages ( 1, 2 ) of the moving bed type operated in series, comprising: feeding of said water/wastewater as a first influent to first stage granular media filters; filtration of said first influent in said first stage granular media filters to produce a first effluent; feeding said first effluent as a second influent to second stage granular media filters; and filtration of said second influent in said second stage granular media filters to produce a second effluent; wherein said second stage granular media filters ( 2 ) are operated with intermittent washing of the granular filter media.

FIELD OF THE INVENTION

The present invention relates to the treatment of water/wastewater, andmore particularly, to an improved method for removingimpurities/pollutants, for example particles, precipitates, metals,emulsions, algae, bacteria, viruses, Protozoa and their oocysts as wellas other microorganisms and related matter from water/wastewateraccording to the preamble of claim 1.

BACKGROUND OF THE INVENTION

Water normally needs to be purified before it is used by municipalitiesand/or industries, and the wastewater produced by municipalities and/orindustries needs to be treated before it is reused and/or discharged.The quality demands for the treated water/wastewater have got stricterover time due to stricter drinking water and environmental regulations.The need to use new sources for drinking water and process water likeseawater and/or polluted surface water and/or treated wastewater furthercreates a need for advanced and reliable treatment methods and/orsystems to produce acceptable water.

New methods and systems are needed both for complete stand alone systemsas well as for specific treatment steps in a chain of treatment steps,for example pre-treatment before membranes for seawater desalination.

Another such system may be a water treatment system where drinking wateror industrial process water is produced from seawater and/or surfacewater and another system may be a wastewater treatment system, whereinthe wastewater needs to be treated so that it can be discharged orreused in industry or in municipalities or for irrigation, as a partsource of drinking water and similar purposes. A further example is thetreatment of lake and/or river water and/or ground water to producedrinking water and/or process water.

Such methods and systems should be simple, reliable and produce treatedwater of a very high quality. Such treatment systems and/or steps shouldfurther be energy efficient and use as little chemicals as possible andproduce as little reject and/or polluting by-products of the treatmentas possible.

In order for such treated water as mentioned in the examples to beuseful, particles, dissolved substances, algae, bacteria, viruses,Protozoa, organic substances, phosphorus and other nutrients, arsenic,metals and other pollutants may in many cases need to be removed to avery high degree from the water/wastewater. Moreover microorganisms,such as

Cryptosporidium and Giardia and their oocysts and/or cysts, may need tobe removed from the water/wastewater. Many systems have one or moresteps that convert dissolved or colloidal matter to particles that canbe separated by solids/liquid separation techniques. In such apurification process and/or as a treatment step in such a purificationprocess, the water/wastewater may be subjected to precipitation and/orflocculation treatment. Dissolved substances such as humic substances,metals, nutrients e.g. phosphorus and/or poisonous substances likearsenic and its compounds, fluorides and/or pesticides are converted tosolid particles and/or absorbed or adsorbed on solid particles. Suchparticles may be of colloidal size and/or created by precipitation andmay need to be flocculated in order to create bigger particles.Colloidal matter and other fine particles present in thewater/wastewater may also need to be subjected to precipitation and/orflocculation treatment in order to create bigger particles.

In this regard, conventional chemical treatment can include chemicalinjection and flash mixing/precipitation followed by one or moreflocculation tanks in which the water/wastewater is agitated withstirrers or agitators in order to create bigger particles, flocs afterwhich it passes through one or more sedimentation basins for separationof particles and/or flocs. One of the disadvantages of conventionalchemical treatment processes is the large area and/or volume requiredfor the flocculation tanks and sedimentation basins. A furtherdisadvantage of conventional chemical treatment techniques is the longresidence time for the water/wastewater in the flocculation tank as wellas in the sedimentation basin.

The use of chemicals addition, flocculation tanks and sedimentationbasins alone in the chemical treatment process does not typically resultin a high enough water purity for many applications. While membranefiltration with a suitably tight membrane can be used to attain a higherlevel of purification, such membrane filters are expensive and haveother disadvantages. On the other hand, a granular media filter, forexample a sand filter, can be added in the treatment chain to increasethe purity of the water being treated. The granular filter media in suchgranular bed filters must be cleaned. In some such filters the granularfilter media is cleaned by being subjected to back-washing at intervalsand/or when the pressure drop over the filter bed has reached apredetermined level. This means that the filtration has to be shut offwhile the granular bed filter is backwashed. Further, during a period oftime after backwashing such granular bed filters produce a firstfiltrate which is of low quality and has to be discharged when highquality water is to be produced. The reason for the low quality of thefirst filtrate is that after back-washing the granular filter bed isclean and free from separated solids. However, such separated solidsassist the separation in the filter bed and therefore their absenceleads to low filtrate quality. Great savings may be obtained if acontinuously operated granular bed filter of the moving bed type isused. The most common type of granular filter media used is filter sand.Such filters are described in U.S. Pat. No. 6,426,005 B1, U.S. Pat. No.4,126,546, U.S. Pat. No. 4,197,201 and U.S. Pat. No. 4,246,102, as wellas in U.S. Pat. No. 5,843,308. The filters described in these patentdocuments are of the moving filter bed type.

In such a filter chemicals may be added to the influent to the filterand precipitation, flocculation and separation can all take place in thefilter bed as described in U.S. Pat. No. 4,246,102 and in U.S. Pat. No.6,426,005 B1. A further advantage is that the continuous granular bedfilter will not have to be taken out of operation for backwashing andthus there will not be a need for extra capacity to take care of theflow that should have been treated in the conventional filters in aconventional plant being backwashed. There will be no first filtrate ina continuous sand filter since the continuous washing is arranged sothat part of the granular media is continuously taken out of the filterbed, washed and returned to the filter bed, so that a steady state isreached where a suitable amount of separated particles is always left inthe granular filter bed. Continuous sand filters are well known todayand there are many plants having continuous sand filters working as apurification step in water treatment systems for municipal andindustrial water and wastewater.

The continuous sand filters generally work well but as mentioned abovethere are applications were really high purity of the treated water isrequired and where e.g. membrane treatment would be required. Membraneplants are however expensive, sensitive and costly to run due to highenergy demand and the necessity to exchange the membranes at regularintervals. Furthermore the cleaning of the membranes normally produceslarge quantities of reject water and in many cases poisonous chemicalsare needed for the cleaning of the membranes. Therefore, there is a needfor more economical treatment systems that are simpler, more robust andmore energy efficient and produce less reject or wash water but stillproduce a very high quality filtrate as the effluent.

An example of such a systems is e.g. disclosed in U.S. Pat. No.6,426,005 B1 relating to a method and system for treatingwater/wastewater including two serial, continuously operating granularmedia filters of the moving bed type with continuous washing of thegranular bed media, e.g. moving bed sand filters. The type of systemdisclosed in U.S. Pat. No. 6,426,005 B1 can either be used as a standalone system for treatment of water and/or wastewater or be used as apart of a treatment system consisting of a number of steps.

The treatment system according to U.S. Pat. No. 6,426,005 B1 thuscomprises two serial continuously operating granular media filters ofthe moving bed type that may have different size of filter media, e.g.filter sand. In addition, chemicals for coagulation/flocculation may beadded and the liquid to be treated may be subjected to a disinfectingtreatment and/or a mechanical, biological and/or chemical treatment. Thewater/wastewater to be treated is introduced as an influent into saidfilters. The water/wastewater is treated within said filters such thattreated, processed water/wastewater or effluent is produced and theimpurities separated in the bed in the first granular media filter aredischarged from the first granular media filter as a first reject. Theeffluent from the first granular media filter is further filtered in thesecond continuously operating granular media filter such that theeffluent from the second granular media filter is the filtrate and theimpurities separated in the bed in the second granular media filter aredischarged from the second granular media filter as a second reject. Inorder to further concentrate the pollutants in the first and secondrejects, the first and second rejects being discharged from the firstand second continuously operated granular media filters, respectively,are introduced into a separate treatment apparatus. In such a treatmentapparatus, the rejects containing pollutants separated from thewater/wastewater being treated in the first and second serial granularmedia filters are subjected to a renewed treatment and/or separatetreatment that eventually results in purified water that meets qualitystandards and a sludge that can be dewatered and/or processed, as itsend products.

However, although this system works very well, and high quality watercan be produced with it, there is still a need to produce even cleanertreated water or effluent. For example such a system cannot from manysources of seawater and/or water from common sweet water sources fordrinking water/process water consistently produce water having a puritymeasured as SDI (Silt Density Index) 4 or lower. Considering that theusual requirement for a pre-treatment step upstream of a reverse osmosismembrane (R/O) is that the SDI value should at all times be equal to orlower than SDI=4 it is of the utmost importance to be able to fulfillthis requirement in an economical and efficient way.

Pure water is a limited resource in the world, while at the same timesea water is an immense resource for drinking water. Both these factorshave influenced companies and inventors to develop methods and means forwater purification and techniques for desalination of sea water. Onetechnique for doing this is reverse osmosis.

Reverse osmosis which uses membranes that are both expensive and verysensitive is a realistic technique for desalination of sea water, but itis essential that the influent to a reverse osmosis plant issubstantially free from particulate matter and other pollutants, orexpressed in other terms, has a silt density index SDI, which does notexceed SDI 4, and preferably is even lower. It is not possible toconsistently reach this high purity level when treating feed water witha substantial variation in the level of pollutants with existingtechniques using granular media filters, and as mentioned above with theadvanced system according to U.S. Pat. No. 6,426,005 B1, only underfavourable circumstances is it possible to obtain an SDI of around 5 orlower.

In the treatment of municipal wastewater a far reaching reduction ofnutrients is required in order to prevent eutrophication of rivers andlakes. Some municipalities demand a level of total phosphorus of 0.02milligrams per litre or lower. This is normally not possible toconsistently achieve by the use of conventional granular bed filterswith chemical treatment and even with a system such as that described inU.S. Pat. No. 6,426,005 B1. Consequently improved systems that arereliable and cost efficient and that can achieve this effluent qualityare highly desirable.

In other applications bacteria, viruses and/or microorganisms likeGiardia and Cryptosporidium and their oocysts need to be removed to thehighest degree. Also here improved systems are highly desirable.

Arsenic in drinking water sources presents another example where highefficiency separation in a simple, robust, and reliable system is muchneeded.

Ground water may contain particles, metals and/or dissolved substancesthat may need to be removed to a high degree.

The examples just represent a few cases of many such applications wherea very high degree of purification is needed and where the method andsystem should be cost effective, simple to operate and reliable.

SUMMARY OF THE INVENTION

One object of the present invention is to provide an improved method forthe treatment of water and/or wastewater which is suitable for allapplications where particles and/or other pollutants and/or emulsionsare to be separated to a very high degree in order to produce a veryclean effluent.

Another object of the invention is to provide an improved method for thetreatment of seawater and/or other salt containing water that makes itpossible to consistently produce water which is sufficiently pure (SDIconsistently below 4) to be treated in a desalination plant working withreverse osmosis membranes, and at the same time provide a robust systembeing able to cope with comparatively low quality influents withcharacteristics that may vary strongly due to circumstances, e.g. stormsleading to sharply increased silt levels, algal growth, constructionand/or shipping activity near the seawater intake.

Another object of the invention is to provide an improved method forproduction from surface water, ground water and/or wastewater ofdrinking water and/or industrial process water that must be of a highquality like e.g. boiler feed water. In such treatment particles, humicsubstances, microorganisms, viruses, arsenic, metals, fluorides,pesticides and a number of other substances may have to be removed.

Another object of the invention is to provide an improved method forremoving particles from water that has been subjected to precipitationdown to very low levels of pollutants, e.g. 0.02 mg/l or lower for totalphosphorus. Other examples where a very high separation efficiency afterprecipitation can be obtained is for the removal of arsenic, metalsand/or fluorides from water as well as the removal of

Giardia and Cryptosporidium and their cysts and oocysts as well as othermicroorganisms. The mentioned applications are just examples and themethod and system of the invention is suitable in all cases where a veryclean water is to be produced from water and/or wastewater containingimpurities and where the impurities are in particle form and/or can betransformed into particle form and/or may be adsorbed or absorbed onactivated granular carbon and/or other adsorbents/absorbents and/orgranular media coated with adsorbents and/or absorbents and/or reactedwith the help of granular catalyst particles and/or granular mediacoated with catalysts and/or other reactants.

A further object of the present invention is to provide a system orplant for the treatment of water which consumes less energy and producesless reject to be taken care of compared with prior art processes.

Another object of the invention is to provide a system or plant thatuses less chemicals to obtain the desired high purity of the effluent.

These and other objects of the invention will be achieved with themethod according to claim 1 with the features defined in thecharacterizing part, and the plant according to claim 14. Developmentsand preferred embodiments of the invention are defined in the subclaims.

The improved method according to the invention for the treatment ofwater or wastewater, having impurities therein, uses two granular mediafilter stages of the moving bed type, operated in series. Intermittentwashing of the granular filter media in the second or downstreamgranular media filter stage according to the invention, verysurprisingly produces a substantial improvement of the effluent whenused on the same influent and the under the same conditions as for thesystem according to U.S. Pat. No. 6,425,005 B1. For instance, whentreating sea water as a pre-treatment for reverse osmosis the effluentresulting from the method and system according to the invention had aSDI of 4 or lower with chemical addition, while the prior art systemoperated on the same type of water and under equal conditions but withcontinuous washing of the second stage granular filter media bedproduced an effluent with an SDI of approximately 5.

Even more surprising, after optimising the filtration system and/orplant according to the invention and using bigger filters a treatedwater of around SDI 3 was obtained without chemical addition. Since thecost of chemicals constitutes a big part of the cost of operation thisleads to a major cost saving. In addition to the improved effluentquality and the savings related to the cost of chemicals theintermittent washing operation in the second stage granular media filterled to a much lower energy consumption and a much lower production ofreject water.

According to one embodiment of the invention, granular media filters inthe second stage are operated with continuous filtration.

According to another embodiment of the invention, the second stagegranular media filters are operated with intermittent filtration andintermittent washing, i.e. the filtration is stopped during theintermittent washing, and water used for washing of the filter media isreplaced with suitably clean water.

According to a further embodiment of the invention the first stagegranular media filters are operated with continuous filtration andcontinuous granular media washing.

According to a further embodiment of the invention the first stagegranular media filters are operated with continuous filtration but withintermittent granular media washing.

According to a further embodiment of the invention the first stagegranular media filters are operated with intermittent filtration andintermittent washing, i.e. the filtration is stopped during theintermittent washing, and water used for washing of the filter media isreplaced with suitably clean water.

According to a further embodiment of the invention, during intermittentfiltration water used for washing of the filter media is replaced withsuitably clean water, preferably effluent from a parallel filter beingin filtration mode.

According to a further embodiment of the invention, each of said firstand second stage granular media filters are washed by removing granularfilter media from the bottom part of the filter bed, transporting it toa media washer, washing it and returning it to the top of the filterbed, while a reject consisting of wash water and pollutants is produced.

According to a further embodiment of the invention granular filter mediais transported from the bottom part of the filter bed with an air liftpump to a media washer, the washed filter media is returned to the topof the filter bed and the reject produced consisting of wash water andpollutants is discharged through a reject pipe.

According to a further embodiment of the invention a valve in the rejectpipe is kept open only during the washing of filter media.

According to a further embodiment of the invention the valve in thereject pipe is opened at a suitable time before starting the air liftpump and is closed at a suitable time after the air lift pump isstopped.

According to a further embodiment of the invention a continuousfractional flow of water through the reject pipe is maintained betweenwashings.

According to a further embodiment of the invention, coarser granularmedia is used in said first stage granular media filters in relation toin said second stage granular media filters. Further, differentdensities of the granular media can be used in the first and secondstage granular media filters, respectively.

According to a further embodiment of the invention, said first stagegranular media filters are arranged with their effluent pipes at ahigher level above ground in relation to the influent pipes of saidsecond stage granular media filters.

According to a further embodiment of the invention a shorter filter bedis used in the second stage filters.

According to a further embodiment of the invention the rejects from thefirst and second filter stages are either discharged or subjected tofurther treatment and/or disinfection in order to concentrate and/ormake harmless the rejects, or they are returned to an earlier step inthe treatment chain. The reject from the second filter stage may as analternative be returned to the influent of the first stage granularmedia filters.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described more in detail in the followingdescription with reference to the accompanying drawings, of which

FIG. 1 schematically illustrates a two stage filtering system withgranular media filter modules of the moving bed type for performing themethod according to the invention, where each of the first and secondstages consist either of any number of free-standing granular mediafilter modules operated in parallel (in case of more than one filter) orconsist of a number of granular media filter modules arranged in filtercells, in which case a filter plant will comprise an arbitrary number ofcells in each filter stage being arranged to work in parallel with eachother (in case of more than one cell).

FIG. 2 schematically illustrates a system for treatment of sea water orsurface water according to the invention and with optional chemicaltreatment

FIG. 3 schematically illustrates the discharge of reject water from thegranular media washer with a reject valve and a bypass on the rejectpipe.

FIG. 4 a schematically illustrates part of a filtration plant accordingto the invention.

FIG. 4 b is a cross-sectional view taken along the line IVb-IVb in FIG.4 a.

FIG. 4 c is a cross-sectional view taken along the line IVc-IVc in FIG.4 a.

FIG. 5 illustrates a prior art water/wastewater treatment apparatusincluding two serial, continuously operated sand filters with continuousfiltration and continuous washing of the granular filter bed media.

DETAILED DESCRIPTION OF THE INVENTION

The prior art filter system according to U.S. Pat. No. 6,426,005 B1 isschematically illustrated in FIG. 5 of the drawings. The workingprinciple is exhaustively explained in the specification of said patent,which is incorporated herein by reference, in its entirety.

In the prior art process described in U.S. Pat. No. 6,426,005 B1, afirst, or upstream filter A works with continuous filtration ofwater/wastewater fed upwards through the granular filter media, whilethe bed of granular filter media moves downwards. A part of the dirtygranular filter media is transported with an air lift pump up to the topof the filter and is washed and returned to the top of the filter bed.In the prior art device the second or downstream filter B worksaccording to the same principle i.e. continuous filtration andcontinuous, uninterrupted washing of the filter media.

FIG. 1 illustrates a filter system for performing the method accordingto the invention. The system comprises a first 1 and a second 2 stagegranular filter media filter of the moving bed type, corresponding tothe filters disclosed in U.S. Pat. No. 6,246,005 B1.

According to the invention, the second stage granular media filter 2 isoperated with intermittent washing of the granular filter media.Granular filter media is transported by an air lift pump 31 from thebottom part of the granular filter media bed to a media washer 5. Thereject pipe 3 is provided with a valve 4 so that the flow of wash waterthrough the media washer can be turned off when there is no washing andconsequently no need for wash water. It is advantageous to arrange thecontrol of this valve so that it opens a suitable time before the airlift pump is started and closes at a suitable time after the air liftpump is stopped. This is to assure that there is a flow of wash wateralready when the first granular media particles enter the media washerwhen the air lift pump is started, and so that all granular mediaparticles in the media washer when the air lift pump is stopped will bewashed. The air lift pump is provided with a control, shown as acontrolled valve 6 in a pipe for compressed air 7 for turning the airflow on and off i.e. starting and stopping the pumping action of theairlift pump 31, and thus together with the valve 4 controlling theintermittent washing of the granular filter media.

By controlling the reject valve 4 and the air lift pump 31, for instanceallowing two washing periods of 30 minutes each per 24 hours theconsumption of pressurized air, and thus energy consumption for washingof granular filter media, in the second stage granular media filter 2will be reduced by more than 95% and the reject volume will be reducedby a factor 20 making it much easier to take care of and/or treat.

In case the incoming water to the first stage granular media filter 1contains a low amount of impurities then it may be advantageous tooperate also the first stage granular media filter 1 with intermittentwashing of the granular filter media.

The reject consisting of wash water and pollutants, discharged throughreject pipe 8 from the first stage granular media filter and throughreject pipe 9 from the second stage granular media filter resulting fromthe two stage filtration according to the invention may be taken care ofaccording to any of the methods described in U.S. Pat. No. 6,426,005 B1,or when the two stage filtration is part of a larger treatment system itmay be returned to an upstream treatment step or alternatively thereject may be discharged directly e.g. into the sea or a lake in caseits composition allows this. The reject from the second stage granularmedia filter can also be returned to the influent to the first stagegranular media filter, either directly upstream of the first stagegranular media filter or upstream of or together with injection ofchemicals. The process and the operation of the two granular mediafilter stages is controlled from a control unit 10.

FIG. 2 shows a large scale two stage granular media filter systemarranged for a bigger plant or as part of a bigger plant with optionalchemicals addition. In the illustrated example, lake, river or sea water11 is taken into a plant through an intake screen 12, through a feedpipe 13. Chemicals can be introduced into the feed pipe 13 from a dosingapparatus 14 directly or upstream of a mixing device, such as staticmixer 15 or a dynamic mixer, before the first granular media filterstage 16. When addition of chemicals is discussed with reference tostatic mixers in the following description, this is one example. Dynamicmixers and direct addition of chemicals can be used instead.

In case chemicals are added precipitation, flocculation and separationcan be carried out in the first granular media filter stage 16 and thetreated effluent from the first granular media filter stage 16 isfurther polished by filtration in a second granular media filter stage17, so that remaining particles and/or flocs can be separated in orderto produce a very clean effluent leaving the second granular mediafilter stage 17 through the pipe 18. In some cases it is advantageous toadd chemicals only to the first granular media filter stage 16, e.g. viathe static mixer 15, to both the first granular media filter stage 16e.g. via the static mixer 15 and the second granular media filter stage17, e.g. via a static mixer 19, only to the second granular media filterstage e.g. via the static mixer 19, or no chemical addition at all. Inmany cases where a system according to the prior art of U.S. Pat. No.6,246,005 B1 requires the addition of chemicals, the system according tothe invention can produce cleaner effluent without any addition ofchemicals which leads to a great cost saving. Reject in the form of washwater containing pollutants separated from the granular filter medialeaves the first granular media filter stage through the pipe 20, andfrom the second granular media filter stage through the pipe 21. Thesereject streams can be taken care of in different ways, which will bediscussed further below.

FIG. 3 shows a media washer 30 used according to the invention forwashing granular filter media taken from the bottom part of the granularfilter media bed and transported with an air lift pump 31, the top ofwhich is shown in FIG. 3. A reject pipe 32 transporting the reject fromthe media washer is also shown.

A valve 33, also discussed in connection with FIG. 1 with the referencenumeral 4, is arranged in the reject pipe 32.

When producing extremely clean water every potential source of pollutionmust be eliminated. One such potential source of pollutants whenoperating a granular media filter of the moving bed type withintermittent filter media washing is the possibility that, between thewashing periods, a small flow of water passes up through the air liftpump tube without being sufficiently filtered in the granular filtermedia bed and escapes into the effluent by moving downward through thefilter media washer.

According to the invention such pollution is prevented by allowing afractional flow of water to pass up through the media washer and outthrough the reject pipe 32 and carrying with it any pollutants that mayhave escaped from the air lift pump. This can be achieved with a bypassconduit 34 arranged around the valve 33. This by-pass is also providedwith a shut-off valve 35. As an alternative, the valve 33 may beconstituted by a valve means which does not close completely or which,when washing is not performed, may be controlled to let a fractionalflow of water pass through the valve.

FIG. 4 a shows a schematic top view, FIG. 4 b shows a cross-sectionalview taken along the line IVb-IVb in FIG. 4 a, and FIG. 4 c shows across-sectional view taken along the line IVc-IVc in FIG. 4 b, of aplant comprising first stage granular media filters of moving bed typeand second stage granular media filters of moving bed type arranged incells each containing four granular media filter modules. The layoutshown in FIG. 4 a has been chosen only for the ease of understanding,and it is obvious that any number of cells can be used containing anynumber of granular media filter modules.

In a concrete structure three first stage filter cells 40 a, b, c, eachcontaining four granular media filter modules 41 a, b, c, d arearranged. A first influent, which could be sea water, lake water, riverwater, ground water or water from a preceding process, enters the plantthrough a feed pipe 42 into a trough 43 communicating with each of thefilter modules through first feed pipes 44 a, b, c. A first effluentfrom the filter cells 40 a, b, c enters a second trough 45, which isconnected to the second stage filter cells 46 a, b, c through secondfeed pipes 47 a, b, c, for supplying this first effluent from the firststage filter cells as a second influent to the second stage filter cellscontaining four second stage granular media filter modules 48 a, b, c,d. A second effluent from these second stage filter cells enters into athird trough 49 and leaves the plant through the outgoing pipe 50.

The second feed pipes 47 a, b, c are provided with valves 51 a, b, c foran optional closing of the supply of second influent into the secondstage filter modules, i.e. intermittent filtration in the second stagefilter cells.

When producing extremely clean water (SDI around 3 or lower) everypossible source of pollution of the effluent has to be avoided. In orderto avoid any risk that particles enter into the effluent from the secondstage granular media filter cells, filtration may be turned off duringwashing and wash water used for washing the granular media can bereplaced with suitably clean water. This can be done from an externalsource, such as through a pipe 52. Alternatively, the wash water can bereplaced with effluent from another filter cell being in filtering mode.This could be accomplished using a tube 53 connecting the effluentvolumes of the cells between two adjacent filter cells, or with the useof pump means 54 a, b. The tube is preferably provided with controlledvalve means 55.

The reason for this arrangement is to prevent that particles that havebeen captured in the granular filter media bed are dislodged by themovement of the media particles during the washing and escape into theeffluent. Even if the effect is small there is an obvious need toeliminate it when producing an extremely clean effluent.

For sake of clarity of FIGS. 4 a, b and c a tube 53 is shown onlybetween the adjacent cells 46 b and 46 c and pump means 54 a, b onlybetween the adjacent cells 46 a and 46 b. The means chosen for thecommunication between cells is preferably mounted between all adjacentcells working in parallel. Further, reject pipes have been left out inFIG. 4 a. In FIGS. 4 b, c is schematically shown reject pipes 56 and 57from the first stage granular media filter modules and second stagegranular media filter modules, respectively. The reject pipe in eachcell is connected to all filter modules in the cell, which isillustrated with pipes 57 a, 57 b in FIG. 4 c. As discussed above, thereject pipes 56, 57 are provided with valve means 58, 59, which arecontrolled from a control unit, which however is not illustrated in theFigs. for the sake of clarity.

Consequently, depending on the circumstances, the filtration maycontinue during the washing, or it can be stopped, that is, in that caseboth the filtration and the washing are performed intermittently.

In the embodiment illustrated in FIGS. 4 a-c, the plant is laid out forcontinuous filtration and continuous washing in the first stage granularmedia filters. However, it is also possible to operate said first stagegranular media filters with continuous filtration and intermittentwashing, and with intermittent filtration, i.e. stopping the filtrationduring the intermittent washing. In this last case, the same methods andtype of means would be used as in the corresponding case for the secondstage granular media filters, using controlled valve means 60 in areject pipe 61, as schematically illustrated in FIG. 1. Replacementwater can be supplied in the form of effluent from parallel filtersbeing in filtration mode, from second stage filters or from an externalsource.

The present invention is not limited to the systems described above butis suitable to use in applications of all kinds where a high purityliquid is to be produced by filtration.

1. A method for the treatment of water or wastewater, having impuritiestherein, by filtration through two granular media filter stages of themoving bed type operated in series, comprising: feeding of saidwater/wastewater as a first influent to first stage granular mediafilters; filtration of said first influent in said first stage granularmedia filters to produce a first effluent; feeding said first effluentas a second influent to second stage granular media filters; andfiltration of said second influent in said second stage granular mediafilters to produce a second effluent; characterized by intermittentwashing of the granular filter media of said second stage granular mediafilters.
 2. The method according to claim 1, characterized in that thesecond stage granular media filters are operated with continuousfiltration.
 3. The method according to claim 1, characterized in thatthe second stage granular media filters are operated with intermittentfiltration and that water used for washing of the filter media isreplaced with suitably clean water.
 4. The method according to claim 3,characterized in that the replacement water consists of water producedas second effluent.
 5. The method according to claim 1, characterized inthat the first stage granular media filters are operated with continuousfiltration and continuous granular media washing.
 6. The methodaccording to claim 1, characterized in that the first stage granularmedia filters are operated with continuous filtration but withintermittent granular media washing.
 7. The method according to claim 1,characterized in that the first stage granular media filters areoperated with intermittent filtration and intermittent washing and thatwater used for washing of the filter media is replaced with suitablyclean water.
 8. The method according to claim 7, characterized in thatthe replacement water consists of water produced as first and/or secondeffluent.
 9. The method according to claim 1, characterized in that thegranular filter media in said granular media filters is washed byremoving granular filter media from the bottom part of the filter bed,transporting it to a media washer, washing it and returning it to thetop of the granular filter media bed, while a reject consisting of washwater containing pollutants is produced.
 10. The method according toclaim 9, characterized in that granular filter media is transported fromthe bottom part of the filter bed with an air lift pump to a mediawasher, that washed filter media is returned to the top of the granularfilter media bed and that reject is discharged through a reject pipe.11. The method according to claim 10, characterized in that a valvemeans in the reject pipe is kept open only during the washing of filtermedia.
 12. The method according to claim 10, characterized in that avalve means in the reject pipe is opened a suitable time before startingthe air lift pump and is closed at a suitable time after the air liftpump is stopped.
 13. The method according to claim 11, characterized inthat a continuous fractional flow of water through the reject pipe ismaintained between washings.
 14. A plant for the treatment of water orwastewater, having impurities therein, for performing a method accordingto claim 1, comprising at least one first stage free-standing granularmedia filter module of the moving bed type or at least one filter cell(40 a, b, c) comprising at least one first stage granular media filtermodule (42 a, b, c, d) of the moving bed type and at least one secondstage free-standing granular media filter module of the moving bed typeor at least one filter cell (46 a, b, c) comprising at least one secondstage granular filter media filter module (48 a, b, c, d) of the movingbed type, operated in series, the effluent from first stage granularmedia filter module (s) or filter cell (s) being the influent of secondstage granular media filter module (s) or filter cell (s), characterizedin that the second stage granular media filter module (s) (2; 48 a, b,c, d) is (are) arranged to be intermittently washed and is (are)provided with controlled valve means (4; 33; 59) for stopping rejectflow between washings.
 15. A plant according to claim 14, characterizedin that the first stage granular media filter modules (1; 42 a, b, c, d)are arranged to be intermittently washed and are provided withcontrolled valve means (60; 33; 58) for stopping reject flow betweenwashings.
 16. The plant according to claim 14, characterized in that thevalve means (33) comprises a by-pass arrangement (34, 35) which admits acontinuous fractional flow of water between washings.